1. Field of the Invention
The present invention relates to an image display technique for improving a moving image resolution.
2. Description of the Related Art
Recently, commercially available moving image display apparatus for television and the like include so-called liquid crystal displays, plasma displays, and FED displays as well as CRTs. Thus, there are various moving image display apparatus. Each type of moving image display apparatus is designed to reduce motion blurring and flicker by N-folding the frame frequency (frame rate) of an input image signal (that is, dividing one frame into N subframes) and then displaying the resultant image.
The user of a display apparatus designed to emit light almost all the time in a frame time, for example, a hold-type display apparatus, observes relatively large motion blurring. When the user pursues (follows an area of motion in a moving image with his/her gaze), he/she observes larger motion blurring with an increase in the period of light emission in a frame time. Flicker tends to be observed in synchronism with frames in a display apparatus, for example, an impulse-type display apparatus, in which the temporal unevenness of light intensity is large because the period of light emission in a frame time is very short.
In contrast to this, when a 60-Hz input image signal is displayed after the frame frequency is doubled (N=2, so-called double-speeding) into 120 Hz, the period of light emission in one frame is reduced to half, and hence the motion blurring is reduced to about half. In addition, with regard to flicker, doubling the frame frequency into 120 Hz can make the frequency of flicker synchronized with frames fall out of the range of the response characteristics of human vision. This can therefore make it difficult to observe flicker.
There are two main methods to increase a frame frequency. The first method estimates an image between two frames by detecting the motion vector of an object in an original image. In general, this method is called, for example, an intermediate image generating method based on motion compensation, which is disclosed in, for example, Japanese Patent Laid-Open No. 2004-159294. The first method is expressed as “frame (field) interpolation based on motion compensation”.
According to the second method, performing filter processing for an input image for each frame will split the image into high spatial frequency components (high-frequency components) greatly associated with motion blurring and low spatial frequency components (low-frequency components) greatly associated with flicker. High-frequency components are concentrated and displayed on one subframe (one of two double-speed frames corresponding to an original frame). Low-frequency components are concentrated and displayed on one subframe or distributed and displayed on the two subframes. As the second method, for example, the methods disclosed in Japanese Patent Laid-Open Nos. 6-70288 and 2002-351382 and U.S. Patent Laid-Open No. 2006/0227249 each are available. In this specification, this second scheme is expressed as a scheme of splitting an image into a plurality of spatial frequency components and displaying the respective frequency components after distributing them to one or a plurality of subframes, and will be briefly expresses as “spatial frequency splitting”.
As shown in FIG. 13, the method disclosed in Japanese Patent Laid-Open No. 6-70288 temporarily stores an input field image in two field memories while switching the field memories, thus forming two double-speed subframes. The speed of an original signal is doubled by alternatively switching these subframes at the rate double the input frequency by using a switch SW0. At this time, this method performs the processing of suppressing high spatial frequency components for one double-speed subframe. As a result, the double-speed subframe having undergone the processing of suppressing high-frequency components (expressed by “SL” in FIG. 13) contains relatively fewer high-frequency components. The other double-speed subframe (expressed by “SH” in FIG. 13) contains relatively more high-frequency components. This makes it possible to localize high-frequency components on one double-speed subframe in an output image.
As shown in FIG. 14, according to the method disclosed in Japanese Patent Laid-Open No. 2002-351382, a frame converter doubles the speed of an input image, and a filter LPF/HPF splits the image into low-frequency components Low and high-frequency components High. In addition, the high-frequency components High are multiplied by a gain α for each double-speed subframe. The sign of α is changed for each double-speed subframe by setting positive α for one double-speed subframe and negative α for the other double subframe. If it is determined that the motion of an image is large, the absolute value of α may be increased. This makes it possible to localize high-frequency components on one double-speed subframe SH.
As shown in FIG. 15, the method disclosed in U.S. Patent Laid-Open No. 2006/0227249 generates high-frequency component data H by applying a filter HPF to an input image. In addition, adding the high-frequency component data H to the input image will generate high-frequency output image data SH. Subtracting the high-frequency component data H from an input image A will generate low-frequency image data SL. Switching these data at a frequency double the frame frequency of the input image using the switch SW0 can output a double-speed image one of whose subframe has localized high-frequency components.
The first method “frame (field) interpolation based on motion compensation” has the problem that since it requires the processing of generating an intermediate image by using the image data of adjacent frames, a large amount of calculation is required. The second method “spatial frequency splitting” has the problem that since displayed images of the first and second subframes do not properly reflect the difference of each time to be displayed, an image lag (tail-blurring) occurs in an area with motion.